The invention concerns electrical pickups for acoustic guitars. Acoustic guitars, which are the traditional form of guitar, produce a significant output of direct sound energy, largely due to the ability of the body of the guitar to pick up and amplify the vibrations of the strings. As a result of this mechanism, the body contributes considerably to the tonal quality of the sound produced by the guitar. Acoustic guitars produce sufficient direct sound output for them to be usable without amplification when played in small rooms in front of small audiences. To be heard in larger auditoriums, amplification is necessary. For amplification to be used, some means for picking up the sound output of the guitar must also be used.
Electrical pickups for acoustic guitars must be distinguished from electrical pickups for electric guitars because the primary mechanism by which each kind of guitar produces sound is quite different. Electric guitars produce sound by using one or more electric coils to pick up the vibration of the strings (which must be of a magnetic material, normally steel) in a magnetic field. The electrical output of the coils is then amplified and the amplified signal is then reproduced by means of a loudspeaker. Electric guitars produce relatively little direct sound energy themselves, and are heavily reliant on amplification if they are to be heard by more than only the player. Unlike the body of an acoustic guitar, the body of an electric guitar contributes relatively little to the direct sound energy output and to the tonal quality of the sound produced by the loudspeaker.
The conventional approach to picking up the sound on an acoustic guitar is to use a microphone mounted on a stand and directed towards the top of the guitar. A microphone works quite well for solo or small ensemble performances of classical music, but presents at least four problems in performances of more popular music: (1) it restricts the player's ability to move around during the performance; (2) it may pick up too much noise from the action of the player's fingers and hands on the strings and top of the guitar (such noise is called "top noise"); (3) it may pick up its own amplified output, leading to acoustic feedback problems; and (4), when the player shares the stage with loud instruments such as drums, keyboards, and electric guitars and basses, it makes achieving the desired sound balance difficult because it picks up sounds from these other sources in addition to sounds from the acoustic guitar. As a result of these problems, there has for a number of years been a tendency towards using self-contained acoustic guitar pickups which allow the acoustic guitar itself to produce an electrical output signal that is fed by a long cable, or a radio-frequency or infra-red transmitter/receiver arrangement to suitable amplification and loudspeaker equipment. Such a self-contained pickup arrangement can solve the problems discussed above.
Because it is desirable not to use steel strings on acoustic guitars, and acoustic guitars therefore lack the fundamental mechanical-to-electrical transducer mechanism of the electric guitar, the considerable amount of art relating to electric guitar pickups is not applicable to acoustic guitar pickups.
Basic requirements for a self-contained acoustic guitar pickup can be stated as follows: (1) the pickup must convert the mechanical vibrations of the guitar strings and body into an electrical signal; (2) the pickup must pick up some top noise, but top noise pick up should not be excessive; (3) the pickup should pick up the sound of the guitar without adding colorations of its own; (4) the pickup (together with any amplification required) should have a high electrical signal-to-noise ratio; (5) the pickup should not pick up hum, buzz and other externally induced noise; (6) the pickup should pick up the output of each string more-or-less equally; (7) it should be easy to install the pickup in the guitar, and should require a minimum of modifications to be made to the guitar itself; and (8) it should be easy to remove the pickup and restore the guitar to pickup-less operation.
A number of acoustic guitar pickups are already commercially available. The FRAP pickup, described in U.S. Pat. No. 3,624,264 uses three ceramic or crystalline piezoelectric transducers orthogonally mounted on three of the walls of a small box-shaped enclosure filled with silicone rubber. The pickup is attached to the body of the guitar by means of a wax or other suitable adhesive. The transducers are arranged so that one transducer detects motion along the x axis, another detects motion along the y-axis, and the third detects motion along the z-axis. The outputs of the transducers are fed in parallel into a buffer amplifier. This pickup meets requirements (1) through (3), (6), and (7) stated above. However, its electrical output is low, so it suffers from signal-to-noise ratio problems; and its ability to pick up equally from all of the strings is dependent on where it is mounted on the guitar. It is often mounted under the bridge near the end of the bridge over which the higher pitched strings pass, so tends to pick up predominantly from the higher pitched strings. This disadvantage can be overcome by using two pickups, one mounted near each end of the bridge. This has the further advantage of offering "stereo" operation, but at the expense of doubling the already high cost of the pickup.
Another approach is the combination piezoelectric transducer and saddle of Baggs, described in U.S. Pat. No. 4,314,495. The saddle is the part of the bridge of an acoustic guitar on which the strings rest. Practical embodiments of the Baggs pickup differ somewhat from the configuration described in the patent. Practical embodiments use six series-connected ceramic or crystalline piezoelectric transducers, one for each string, encapsulated in epoxy resin in a U-shaped brass channel transducer housing. The transducer housing is an integral part of a saddle formed using a fibre/resin material such as that sold under the trademark Micarta. The channel construction of the transducer housing together with the suspension of the piezoelectric transducers in epoxy resin, is thought to reduce top noise (Requirement 2 is met).
Installing a Baggs pickup in a guitar requires that the normal saddleslot in the bridge be machined to increase its width to 1/8" (3.2 mm) and its length to 2.875" (73 mm). Thus, requirement (6) is not met. The changes to the saddle slot mean that if the pickup is removed, it must be replaced by a non-standard saddle. Thus, requirement (7) is not met. Moreover, since the pickup includes a completely new saddle, the guitar must be re-intonated when the pickup is installed. Finally, the brass insert in the Baggs pickup makes it more rigid than a normal saddle, which changes the playing action of the guitar. Adjustments to the shape of the saddle are required to restore the action to normal. The pickup is also relatively short lived: the plastic saddle wears considerably more quickly than a conventional bone saddle and, when the saddle wears out, the whole pickup must be replaced. Bone cannot be substituted for plastic because it does not have appropriate directional characteristics (see below). The plastic saddle also tends to break off the brass transducer housing. Each time a saddle wears out or breaks, a new pickup must be installed and the guitar re-intonated.
The Baggs pickup also has some inconvenient electrical properties. The plastic material used in the saddle enables the transducer mounted under each string to pick up vibrations from its own string much more efficiently than vibrations from adjacent strings. The pickup exploits this property to reduce top noise by connecting the transducers under the A and D strings out of phase with the transducers under the other four strings. However, this arrangement causes phasing problems when the electrical output of the guitar is mixed with any signal that might include a component representing the acoustic output of the guitar.
The Fishman pickup is described in U.S. Pat. Nos. 4,727,634, 4,774,867, and 4,944,634. This pickup uses six small (1/16" dia..times.0.02," 1.6 mm dia..times.0.5 mm) cylindrical ceramic piezoelectric transducers, one for each string. The pickup fits in the bottom of a standard 3/32" (2.4 mm) wide saddle slot, and can be used with the existing saddle if about 1/16" (1.6 mm) is removed from the bottom of the existing saddle. This pickup is easy to install, and does not require that the guitar be re-intonated, but it suffers from the general defects of pickups based on ceramic or crystalline piezoelectric transducers discussed below. Moreover, the pickup is quite complex, since it requires separate components to mount the individual transducers resiliently, to interconnect them, and to screen them from outside interference.
All acoustic guitar pickups based on ceramic or crystalline piezoelectric transducers suffer from a number of common problems: (1) such transducers have mechanical resonances in the audio frequency range that colour the sound of the guitar; (2) the mechanical mountings of such transducers have their own mechanical resonances in the audio frequency range that further colour the sound of the guitar; and (3) such transducers are small and are thus awkward to handle in such assembly operations as attaching wires to them, etc.
A new form of piezoelectric material, a polarized homopolymer of vinylidene fluoride (PVDF), has recently become available. This material is sold under the trademark "KYNAR." Full information about this material can be found in the KYNAR Piezo Film Technical Manual (Pennwalt Corporation, 1987). This piezoelectric material is a plastic film which is available in a number of thicknesses (e.g., 28, 52, 110 microns). PVDF film has a number of properties that make it advantageous for use in acoustic guitar pickups: it has a high output voltage for a given mechanical stress; it has a low mass and a low Q, which means that it responds instantly to a mechanical input, and introduces little coloration of the sound.
Electrical contacts can be made to the surface of the film itself by painting electrodes on the surface of the film with conductive paint, or, preferably for mass-production, silkscreening electrodes on the surface of the film with conductive ink, or vacuum depositing metal electrodes on the surface of the film. Attaching leads to the electrodes presents problems, however, because of the material's low softening point and low resistance to tearing. The manufacturer suggests that a low-temperature solder can be used. This enables a reliable electrical contact to be made, but does not result in a mechanically strong attachment between the electrodes and the output lead.
The use of PVDF film as an acoustic guitar pickup is described at page 43 of the KYNAR Technical Manual. A piece of 28 micron thick film, about 3" by 1" has electrodes on both sides. It is electrically shielded on both sides by means of a metallic foil and mechanically protected by a layer of a flexible plastic laminate. Electrical contacts are made (the manual does not say how) to the electrodes on each side of the film. The complete transducer is attached to the top of the guitar, close to the sound-hole, and oriented with its long axis running in the direction of the strings so that pickup of top noise is reduced. The sound of the pickup is influenced by what is used to attach the pickup to the guitar (double-sided adhesive tape is suggested in the Technical Manual). Moreover, this type of pickup tends to pick up strings that are closer to the pickup more efficiently than strings that are more distant. The pickup placement suggested in the Technical Manual would therefore tend to give a bass-heavy output. This problem could be partially solved by using two pickups, one at each end of the bridge, in a "stereo" arrangement.
A practical embodiment of this pickup solves the lead attachment problem by using sprung mechanical contacts to pick up the electrical output of the transducer. This results in a bulky arrangement, compared with the rest of the pickup, the contact device being a flat rectangular box about 1.2.times.1.2.times.0.2 inches (30.times.30.times.5 mm).
An alternative form of acoustic guitar pickup using PVDF film is described in Kynar Piezo Film News, No. 1 (Pennwalt Corp., 1987) at page 4. The sides and bottom of standard-sized saddle are partially wrapped with a piece of PVDF transducer film about 2.8.times.0.7 inches (71.times.18 mm). The long sides of the transducer film are curved to match the curvature of the top of the saddle. The material is metallized completely on the outside and metallized in six segments, one for each string, on the inside (i.e., the side closer to the saddle). The transducer is glued directly to the saddle. There is no mechanical protection or electrical screening; the player's hand can induce an objectional buzz into the output of the pickup if it gets too close to the pickup. This pickup is also relatively short lived: the saddle material is not as durable as bone, the material normally used for making saddles, and the whole pickup must be replaced and the guitar re-intonated, if the saddle wears out.
This basic assembly would install directly in a standard saddle slot without any modification were it not for the large plastic connector assembly on one end of the modified saddle. To accommodate the connector assembly, the width of the saddle slot in the bridge must be increased to about 0.22" (5.6 mm) for a length of about 0.3" (7.6 mm) and the length of the saddle slot must be increased by about 0.07" (1.8 mm). This pickup is therefore inconvenient to install, and difficult to replace if no longer desired.
Practical embodiments of this pickup are sold as part of the Gibson.TM. Symbiotic Oriented Receptor System (S.O.R.S.).
In his copending application Ser. No. 681,116, of which application this application is a continuation-in-part, the applicant described a new configuration of acoustic guitar pickup using PVDF or a similar piezoelectric plastic film transducer element that can be installed in an acoustic guitar without the need to modify the standard saddle slot. The prior application described a number of variations on a basic design that consisted of only four component parts: a piezoelectric transducer element, a core, a contact strip, and an output lead, which was preferably coaxial. The core was elongated, had a plurality of faces at least one of which, preferably the largest, was conducting. Preferably, the core had a rectangular cross-section. In the preferred embodiment of the invention described in the prior application, a piece of piezoelectric film considerably larger than the largest face of the core had a first electrode on one side, the electrode having substantially similar dimensions to those of the conductive face of the core, and had a second electrode covering substantially all of the other side. The first electrode was placed in contact with the conducting face of the core and the film was then wrapped 1 and 1/4 times around the core and secured in place with a conducting adhesive. The contact strip was secured to part of the second electrode on the film. One conductor of the output lead was secured to the conducting face of the core, the other to the contact strip. The wrapped construction of this pickup enabled the piezoelectric film and its two electrodes to serve as the piezoelectric transducer element of the pickup, as the electrical insulator of the pickup, and as the electrical shield of the pickup.
Although the preferred embodiments of the pickups described in the prior application are compact, simple, and have a satisfactory signal-to-noise ratio, their electrical output level is low compared with competing acoustic guitar pickups. It would be difficult to increase the electrical output of the preferred embodiment of the prior pickups by increasing the thickness of the piezoelectric film because thicker films are difficult to bend in the small radii required. Moreover, the several layers of conducting adhesive used in the prior pickups cushion the piezoelectric transducer element and reduce its electrical output. Although the thickness of the piezoelectric film used in the simpler embodiments of the pickups described in the prior application could be more easily increased, these embodiments had inadequate electrical shielding and insulation.